1. Field of the Invention
The present invention relates to a method of sensing a position of a movable body and an apparatus therefor, and more particularly to improvements in a method of sensing a position of a movable body by directly or indirectly converting a unit movement of the movable body into digital pulse signals and an apparatus therefor, suitable for sensing that a workpiece and a tool have reached predetermined relative positions so as to switch the control of approach means, particularly for use in a machine tool, industrial machine or the like.
2. Description of the Prior Art
Since the approach speed of the tool and the like are controlled in accordance with the positional relationship between a workpiece and a tool, i.e., the progress of working in tool machines and industrial machines in general, it is very important to accurately sense the position of the tool or the like. Consequently, heretofore, as shown in FIG. 1 for example, in a machine tool including: a slide base 14 fixedly provided for working a workpiece 12 fixed on a jig 10; a slide table 18 supporting spindles 16 and movable in the forward and backword directions on the slide base 14; a DC motor 24 for moving the slide table 18 forward or backword by rotating a feed shaft 22 threadably coupled to a feed nut 20 solidly secured to the undersurface of the slide table 18; a DC motor driving circuit 26 for controlling the rotational direction and the rotational speed of the DC motor 24; a speed change setting circuit 28 for supplying a setting value of an approach speed properly suitable for the forwarded positions of the spindles 16 to the DC motor driving circuit 26; and a mechanical sequence circuit 30 for controlling the DC motor driving circuit 26; the slide table 18 is provided at predetermined positions on the side surface thereof with dogs 32 and the slide base 14 is provided at predetermined positions thereof with mechanical limit switches 34 adapted to be ON-OFF operated by the dogs 32, whereby the limit switch 34 is ON-OFF operated by the dogs 32 to sense the forwarded position of the slide table 18, i.e., the spindles 16, so that the DC motor driving circuit 26 can be controlled through the mechanical sequence circuit 30, thereby enabling to control the approach of the slide table 18. In the drawings, designated at 35 is a bearing, 36 a stopper solidly secured to the front end face of the slide table 18, 37 a dead stopper solidly secured to the inner wall surface of the slide base 14, CR.sub.1 and CR.sub.2 normal-opposite rotation switching contacts for switching the rotational direction of the DC motor 24, and LS.sub.1.about.LS.sub.4 approach speed changing contacts for changing the approach speed in accordance with the progress of working.
This method has such characteristic features that it is very simple to effect the method and it suffices to provide the mechanical limit switches 34 and dogs 32, both of which substantially correspond in number to required signal outputs at positions where controls are required to be switched. However, the mechanical limit switches are low in mechanical strength and not satisfactorily protected from the atmospheric conditions at the sites of factory such as oil, water, dust and the like, and hence, tend to malfunction due to the presence of chips, dust, cooling water and the like, thus lacking in reliability. Furthermore, the positional relationship between the limit switches and the dogs tends to go out of order with age and it is difficult to adjust the dogs. Further, there is little disadvantage in the use of a single purpose machine in which a single workpiece is worked on by use of a single spindle. However, in the use of a general purpose machine having been demanded in recent years in which a plurality of workings are effected on a single workpiece or a plurality of workpieces, there is presented such a disadvantage that the positions of the dogs and limit switches should be changed each time the workpieces or spindles are replaced with new ones, thus lacking in general-purpose properties.
With the machine tools as described above, in the case position sensing singles emitted from the limit switches are directly used in controlling the approach means or the like, the position sensing signal should be emitted not only at the instant the slide table reaches the setting position but also should be emitted when the slide table has reached the setting position and stays within a predetermined stroke from the setting position, the stroke being suitable for controlling the approach. Consequently, in the case point dog is used for switching the limit switch 34 ON for a very limited predetermined period of time for example, heretofore, a self-sustaining circuit has been additionally used so as to sustain the position sensing signals emitted from the limit switches for a predetermined period of time corresponding to a predetermined stroke, or another point dog has been provided at a terminal point of the predetermined stroke so as to ON-OFF operate a single limit switch by use of two point dogs. As a result, the relay sequence has become complicated, reliability has been lowered, and moreover, the costs for manufacture have been increased. On the other hand, in the case a long dog is used which can operate the limit switch 34 for a comparatively long predetermined stroke, there are encountered such disadvantages that not only the long dog is decreased in ridigity but also it is difficult to adjust the long dog, and the change in signal sustaining time requires the long dog to be replaced. The abovedescribed disadvantages are true of the case a proximity switch is used of the limit switch.
On the other hand, with numerically controlled machine tools and the like which have recently been manufactured, digital servomechanisms each provided therein with a positional feedback system have been adopted in many cases. As shown in FIG. 2, this digital servomechanism includes: a pulse generator 40 for directly or indirectly transducing a movement of a movable body 38 such as a slide table into a digital pulse signal; a reversible counter 42 for reversibly counting pulse signals emitted from the pulse generator 40 corresponding to the position of the movable body 38 and supplying a deviation between preset command signal pulses and the pulse signals thus reversibly counted; a D/A converter 44 for converting an output from the reversible counter 42 into an analog signal; and a servoamplifier 46 for amplifying an output from the D/A converter 44 and supplying an output to a servomotor 48 for controlling the position of the movable body 38. The digital servomechanism as described above can monitor the position of the movable body 38 from time to time, and hence, it should lead to that the approach control with a high degree of accuracy can be effected in the ideal condition. However, in actual practice, in the abovedescribed servomechanism, the parts are large in number, the construction is complicated, the manufacturing costs are very high, the components are low in reliability and backlash and the like in transmission system contribute to unstable operation of the servomechanism. Further, there is presented such a disadvantage that a considerable scale of reconstruction is required before the servomechanism having the positional feedback system can be fitted to the conventional machine tool.